This invention relates generally to an optical element, such as a membrane or pellicle, and more particularly to an optical pellicle with superior properties of transmissivity.
In recent years, pellicles have played an important role in the fabrication of semiconductor wafers used in integrated semiconductor circuits. As is well understood by those in the art, pellicles protect photomasks which are used in the various photolithography steps required in wafer preparation and fabrication.
The typical pellicle takes the form of an extremely thin optical membrane which is supported on a ring-like frame, the entirety of which is then placed over the photomask to prevent contamination of the mask during photolithography. To be most effective, a pellicle should not only prevent contamination of a photomask, but it also must exhibit a very high degree of optical transmissivity at the wavelength of light used during photolithography. The requirements of transmissivity for an optical pellicle are generally discussed in an article entitled "Pellicle Protection of IC Masks" by Ron Hershel, published in 1981 by the Society of Photo Optical Instrumentation Engineers.
Mercury lamps have typically been used as a light source in lithography operations. These lamps normally exhibit maximum amounts of light at wavelengths of 365, 405 and 436 nanometers, known by those skilled in the art as the mercury atom's I, H and G spectral lines ("the I, H and G lines"), respectively. Projection steppers used in lithography have traditionally used mercury's G line, i.e. light having a wavelength of 436 nanometers. For this reason, pellicles have been developed which exhibit maximum transmissivity at the G line. However, to facilitate use at various wavelengths, so-called broadband pellicles have been developed which exhibit an average transmissivity of about 92% over the I, H and G lines.
Because of the shorter wavelengths at the I and H lines, use of these wavelengths from the mercury lamp can increase resolution beyond that achieved at the G line. While the transmission of a conventional 2.85 micron thick pellicle, that is, 92%, is often sufficient, it would be preferable if the transmission was greater than 92% at the I, H and G lines. Of course, any loss in transmission as the light passes through the pellicle will reduce the light which contacts the mask. This requires either stronger mercury lamps, or increased time of exposure.
Pellicles with multiple anti-reflective coating layers have been developed to increase transmissivity. Such pellicles are disclosed in my U.S. Pat. No. 4,759,990. While these pellicles permit higher average transmissivity for the I, H and G lines, they are typically more expensive than pellicles having a single anti-reflective coating on each side.
A necessary feature of all optical pellicles is that they be relatively durable. If a pellicle product breaks during use, it not only requires replacement, which can be a sensitive operation potentially resulting in contamination of the mask area, but the breakage of the membrane itself can cause contamination by pellicle debris falling onto the mask. Thicker pellicles, such as 2.85 micron pellicles sold by Micro Lithography, Inc., are far more durable than thinner pellicles such as those of 0.86 micron thickness. However, with thicker pellicles, it is often more difficult to control pellicle thickness and uniformity. Thickness control directly affects uniformity, and uniformity can vary not only from pellicle to pellicle (or lot to lot) but also across the face of any particular membrane.
It is therefore a general object of the present invention to provide an optical pellicle which overcomes the drawbacks and limitations of prior art proposals. More specifically, the invention has as its objects the following: (1) to provide a pellicle having peak transmissivity at wavelengths corresponding to the wavelengths emitted by a mercury lamp in a photolithography process; (2) to develop a maximum-transmissivity, optical pellicle that is sufficiently durable that it is not prone to breakage, but which is not so thick that lot-to-lot and edge-to-edge uniformity is a problem; (3) to provide a pellicle which can be used with steppers that are used at the higher resolution, shorter I and H lines; (4) to provide a pellicle which can be used interchangeably with a wide variety of steppers using different wavelengths from mercury light; (5) to provide a pellicle which exhibits greater than 92% transmissivity at the critical wavelengths; (6) to develop a pellicle having a single anti-reflective coating on each side which has transmissivity at the above-identified critical wavelengths approaching that of a pellicle having multiple anti-reflective coatings on each side; and ( 7) to provide a superior optical pellicle which can be fabricated using largely proven techniques.